As is well known, as an electric driving apparatus that drives a driving subject by use of a motor, which is a driving source, there exists, for example, an electric power steering apparatus mounted in a vehicle. In the case of this electric power steering apparatus, a control unit drives a motor in such a way that the motor generates output torque corresponding to steering torque exerted on the steering by a vehicle driver, and then the output torque of the motor is exerted on the steering shaft so as to assist the driver's steering.
In general, the control unit for controlling the motor of the electric power steering apparatus usually includes a three-phase inverter circuit The three-phase inverter circuit is provided with U-phase, V-phase, and U-phase high-voltage-side switching devices and U-phase, V-phase, and W-phase low-voltage-side switching devices; in each of the phases, the high-voltage-side switching device and the low-voltage-side switching device are connected in series with each other so as to form a switching arm for each phase. Then, the high-voltage-side switching device in the switching arm for each phase is connected with the high-voltage-side terminal of a DC power source; the low voltage-side switching device in the switching arm for each phase is connected with the low-voltage-side terminal of the DC power source, i.e., the ground potential portion of a vehicle. The each-phase output terminal of the three-phase inverter circuit is led out from the series connection point between the high-voltage-side switching device and the low-voltage-side switching device in the each-phase switching arm.
Meanwhile, the motor of the electric power steering apparatus is formed of, for example, a three-phase brushless motor; each-phase input terminal of the three-phase stator windings provided in the stator is connected with the corresponding each-phase output terminal of the three-phase inverter circuit. In general, such a three-phase motor is provided with a rotor having two or more magnetic poles that are each formed of a permanent magnet.
The high-voltage-side switching device and the low-voltage-side switching device of the each-phase switching arm in the foregoing three-phase inverter circuit are on/off-controlled based on a predetermined pattern and output three-phase electric power through the foregoing output terminal. When its stator windings are energized by the three-phase electric power outputted from the three-phase inverter circuit, the motor of the electric power steering apparatus generates a rotating magnetic field so as to rotate the rotor having two or more magnetic poles, so that output torque, as a predetermined assist torque, is generated from the output axle of the rotor.
In the case of an electric power steering apparatus to be mounted in a vehicle, it is required to take plentiful measures for a failure in terms of securing the safety of a vehicle. An electric power steering apparatus sensitively responds to a torque ripple in the output torque of the motor; therefore, in terms of the comfortability of a drive, it is required that the output torque of the motor is smooth without pulsating.
As an example of conventional electric power steering apparatus, there exists an electric power steering apparatus in which two groups of stator windings are provided in a single motor and in which there is provided a control unit having two respective inverter circuits that can independently control the two groups of stator windings. The control unit in the conventional electric power steering apparatus has two Micro Processing Units (referred to as MPU, hereinafter), each of which has a Central Processing Unit (referred to as a CPU, hereinafter), and drives these MPUs in such a way that they collaborate with each other so as to control the two inverter circuits; when an abnormality occurs in one of the two inverters, only the other one of the two inverters, which operates normally, continues to drive the motor; moreover, in order to prepare for a failure, the conventional electric power steering apparatus has a double-system and redundant control system in addition to the two inverters (for example, refer to Patent Document 1).
As described above, an electric power steering apparatus sensitively responds to a torque ripple in the output torque of the motor; therefore, it is required that the current-detection accuracy of a current sensor for detecting an electric current that flows in the stator winding of the motor (simply referred to as a motor current, hereinafter) is raised so that high-accuracy feedback control is applied to the motor current. However, in the three-phase inverter circuit, the On time of the low-potential-side switching device of the phase, out of the three phases, in which a maximum current flows maybe shorter than the current-detection time of the current sensor and hence no sufficient-accuracy current detection value may be obtained from the current sensor.
Accordingly, by use of the fact that the sum of the respective currents of the three phases is “0”, the current detection value of the phase in which the maximum current flows may be estimated based on the respective current detection values of two other phases. However, when the switching device of the phase in which the maximum current flows performs switching, noise caused by the switching may deteriorate the accuracies of the current detection values of two other phases. Therefore, there has already been disclosed a technology in which when the respective currents of two phases other than the phase in which the maximum current flows are detected, the high-potential-side switching device and the low-potential-side switching device of the phase in which the maximum current flows are held to be on and off, respectively, so that the effect of the noise, caused by the foregoing switching, on the current detection values of the two other phases is reduced (for example, refer to Patent Document 2).
In the case where in an AC rotating electric machine having two or more groups of stator windings, there exists an mutual inductance between the two or more groups of stator windings, the current in one of the stator windings provides an effect to the current in the other one of the stator windings; thus, the respective currents in, voltages across, and torque values of the stator windings are liable to become oscillatory.
Accordingly, for example, there has been disclosed a conventional motor controller in which in the rotating two-axis coordinate system of an AC rotating electric machine, an average voltage command value is obtained based on the difference between an average current command value and the average value of the respective output currents of two or more groups of inverters that are connected in parallel with one another and a difference voltage command value is obtained based on the difference between the respective output currents of the inverters and a difference current command value, in which the average voltage command value and the difference voltage command value in the rotating two-axis coordinate system are restored to the voltage command values for the stator windings, and in which based on the voltage command values, the respective voltages of the stator windings are controlled so that an unbalanced current is reduced (for example, refer to Patent Document 3). In the conventional apparatus, the respective voltages across the windings are controlled based on the respective average current values and the respective difference current values of the two or more groups of inverters, so that interference between the winding groups are prevented.